1. Field of the Invention
The invention relates generally to drill bits for drilling boreholes in subsurface formations. More particularly, the invention relates to methods for designing drill bits, methods for evaluating cutting structures for drill bits, and methods for optimizing a cutting arrangement for a drill bit. The invention also provides a novel method that can be used to calculate scores for cutting arrangements proposed for drill bits.
2. Background Art
FIG. 1 shows one example of a conventional drilling system used in the oil and gas industry for drilling wells in earth formations. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is a drill bit 20. The drill bit 20 is designed to break up and gouge earth formations 16 when rotated on the formations. 16 under an applied force. Formation 16 broken up by the drill bit 20 during drilling is removed from the well bore 14 by drilling fluid typically pumped through the drill string 12 and drill bit 10 and up the annulus between the drill string 12 and the well bore 14.
One example of a conventional drill bit is shown in FIG. 2. This type of drill bit is typically referred to as a roller cone drill bit. The drill bit 20 includes a bit body 22 having a threaded section 24 at its upper end for securing to the drill string (12 in FIG. 1) and a plurality of legs 25 extending downwardly at its lower end. A frustro-conical rolling cone cutter (hereafter referred to as roller cone 26) is rotatably mounted on each leg 25 by a bearing shaft pin which extends downwardly and inwardly from each leg 25. Each of the roller cones 26 has a cutting structure comprising a plurality of cutting elements 28 arranged on the conical surface of the cones 26. The cutting elements 28 project from the cone body and act to break up earth formations at the bottom of the borehole when the bit 20 is rotated under an applied axial load. The cutting elements 28 may comprise teeth formed on the conical surface of the cone 26 (typically referred to as milled steel teeth) or inserts press-fitted into holes in the conical surface of the cone 26 (such as tungsten carbide inserts or polycrystalline diamond compacts).
Many prior art roller cone drill bits have been found to provide poor drilling performance due to problems such as “tracking” and “slipping.” Tracking occurs when cutting elements on a drill bit fall into previous impressions formed in the formation by cutting elements at a preceding moment in time during revolution of the drill bit. Slipping is related to tracking and occurs when cutting elements strike a portion of previous impressions and slides into the previous impressions.
In the case of roller cone drill bits, the cones of the bit typically do not exhibit true rolling during drilling due to action on the bottom of the borehole (hereafter referred to as “the bottomhole”), such as slipping. Because cutting elements do not cut effectively when they fall or slide into previous impressions made by other cutting elements, tracking and slipping should be avoided. In particular, tracking is inefficient since there is no fresh rock cut, and thus a waste of energy. Ideally every hit on a bottomhole cuts fresh rock. Additionally, slipping should also be avoided because it can result in uneven wear on the cutting elements which can result in premature failure. It has been found that tracking and slipping often occur due to a less than optimum spacing of cutting elements on the bit. In many cases, by making proper adjustments to the arrangement of cutting elements on a bit, problems such as tracking and slipping can be significantly reduced. This is especially true for cutting elements on a drive row of a cone on a roller cone drill bit because the drive row is the row that generally governs the rotation speed of the cones.
Currently, cutting arrangements, such as the arrangement of cutting elements on rows of a roller cone drill bit are designed either by gut feel, in reaction to field performance, such as the addition of odd pitches to alleviate tracking and slipping, or by trial and error in conjunction with other programs used to predict drilling performance. The problem in these design approaches is that the resulting arrangements are often arrived at somewhat arbitrarily, which can be time consuming in the evolution of the bit design and may or may not lead to drill bits producing desired drilling characteristics.
Therefore, methods for predicting drilling characteristics prior to the manufacturing of drill bits are desired to reduce costs associated with designing bits and to enhance the development of longer lasting bits and/or bits which more aggressively drill through earth formations. Methods are also desired to minimize or eliminate the design and manufacturing of ineffective drill bits which exhibit significant tracking or slipping problems during drilling. Methods are also desired to reduce the time required for designing effective drill bits. Additionally, drill bit designs that exhibit reduced tracking and slipping over prior art bit designs are also desired.